The force also found boxes of notebooks and other documents that cover the period of the disaster, which could contain vital details.
Deputy chairwoman of the IPCC, Deborah Glass, said: “This is an ongoing criminal investigation, the like of which has never been seen before in this country. Already we are uncovering more about the disaster and its aftermath.
“Hillsborough has had a history of inquiries by the police and others, many completed quickly, coming to flawed conclusions. Our investigations need to deliver the last, definitive account.”
Last month the IPCC revealed that at least one officer made a note of what happened that day, against instructions, and that none of the previous inquiries into the tragedy had recovered any such notebooks.
The disaster, that claimed the lives of 96 Liverpool fans in 1989, is now at the centre of the biggest ever inquiry into police conduct in the UK.
Appeal for witnesses
Investigators are set to appeal for witnesses next week in relation to how West Midlands police ran their inquiry into the handling of the disaster by South Yorkshire police.
Around 12,000 people spoke to West Midlands police as part of their inquiry.
Ms Glass said: “This appeal, which will be launched next week, forms a crucial element to our investigation into how West Midlands police conducted its inquiries into the disaster. We want to hear people’s experiences of that process.
“We have already had a number of people contact us with concerns that their statements were amended and we have no doubt there are others who have not contacted us: we want to be able to present as full as possible a picture of witness evidence, both for the inquests and the criminal investigations.”
Liverpool Football Club is expected to promote the appeal during its home match on 21 September.